Modern water tube boilers are designed by Foster Wheeler on the concept of D type boiler. Those currently in service for marine propulsion purposes are ESD I, ESD II, ESD III, ESD IV, and ESRD (reheat) type boilers. We will be covering ESD series boilers in this article.

Concept of D-type boiler

In the D-type boiler, the superheater is positioned in between/adjacent to the Generating tubes. As the name of the boiler suggests, it has D-shaped bent tubes for heat transfer. Normally the generating tubes are 32 mm in diameter. Screen tubes are placed in between the superheater and the furnace. So, the screen tubes absorb high temperature from the furnace gases and bypass reasonable gas temperature to the superheater. Therefore, the superheater is prevented from overheating. There are two rows of screen tubes, each tube of 50 mm diameter.As the superheater is placed near the generating tubes, cleaning and maintenance become difficult. As the maintenance work is challenging, good quality of feedwater is required for heat transfer to avoid the formation of scales/deposits. Even the replacement of tubes becomes difficult in this condition.

ESD Boilers: Foster wheeler is the company that introduced D type of boilers in the market. Subsequently, these boilers are named after this company and came into existence as Foster Wheeler ESD boilers. The Manufacturing of the ESD [External superheater D type] boiler was taken to overcome the drawbacks of the D type boiler. The superheater is positioned directly beneath the economizer, thus forming an external vertical convection unit at the side of the boiler. This is very accessible for maintenance and water washing. Multiple stages of superheater and usage of Attemperator were possible in this kind of boiler. Before going into detail, let us look into some important tubes and their uses.

• Generating tubes: Numerous small diameter tubes are placed in the main flow of hot gases, so forming a large heat exchange surface. The generation of steam takes place mainly by convection. For a given rate of water circulation, the minimum allowable tube diameter is limited.

• Screen tubes: These tubes receive heat from the flame together with heat from the hot gases leaving the furnace. Therefore, they are relatively large in diameter to keep the ratio of steam to water low enough to prevent overheating. They prevent the superheater from overheating.

• Water wall tubes: These tubes are used to contain the heat of the furnace, thus reducing the amount of refractory material required.

• Superheater tubes: These are small diameter tubes placed in the main gas stream, after the screen tubes. They have to superheat the saturated steam leaving the drum to a temperature suitable for the use of the main turbine. They must be protected from direct radiant heat as they are liable to overheating due to the much smaller specific heat of steam compared to that of water.

Models of Foster Wheeler Boilers

Foster wheeler ESD I boiler

The ESD I or external superheater D-type design is based on that of the D-type boiler, but with the superheater located after the main generating tube bank in the direction of the gas flow. It is situated below the economizer section. It is also provided with an air attemperator between the first and second stage superheater, for control of the final steam temperature. They also reduce slagging and the corrosion of support of superheaters, as it is located a few rows away from the furnace as shown in the below figure.

Feedwater enters via the boiler inlet and passes multiple stages of Economiser sections to raise the temperature of the feed water. Later the feed water enters the steam drum and gets circulated to various down comers and water drum via headers. The heat generated from the furnace gets transferred to the water using convection. The generating tubes act as the heat transfer surface. As the steam being less dense than water, it rises and accumulates at the top of the steam drum. Now the steam enters the air-cooled attemperator after passing through the stage 1 superheater. The air attemperator, consisting of a bank of finned tubes is located in the combustion air duct. Simple interlocked air by-pass and shut-off dampers in the attemperator cool the steam and thus controlling the final steam temperature. This control may be either thermostatically or manually operated. Now the steam enters the final superheater stage and goes out for its various use. Refer to the below Line diagram to understand the steam generation and circulation inside the boiler.

Foster wheeler ESD II boilers

The air-attemperator method of superheat control was found to have some limitations in practice, and to save space, steam piping, weight, fan power, and initial cost, the ESD design was advanced a stage further by the introduction of the ESD II boilers. In the ESD I design the heat input to the superheater increased with load and any excess was removed by an air attemperator. In the ESD II boilers, the heat input to the superheater is limited to the amount of superheat required, this being implemented by providing the superheater itself with an outlet damper, and also a damper-controlled by-pass. So, the superheater itself controls the final superheat temperature, with the help of its dampers.

There is a control unit fitted in the up-flow direction of the gas. This unit is the extension of the economizer section. They absorb heat from the gas and transfer it to the inlet feed water. The feedwater passes the control unit and then enters the steam drum. This happens only when the superheater damper is in closed condition. When the damper is in open condition, the heat from the gas will be transferred to the outgoing steam to raise its temperature. In this way, the control unit and dampers control the temperature of feedwater and final superheat temperature respectively. Refer to the below flow diagram.

The basic design consists of two drums, the larger steam drum being placed above a smaller water drum. The gases leaving this furnace enter a split gas passage. One for the control unit and the other for the superheater section. The dampers regulate the gas flow and so control the final superheat temperature. The furnace has water walls consisting of close-pitched 50 mm diameter tubes at the roof, side, and rear. The lower headers for these water walls are of rectangular section and are supplied with water from the water drum utilizing underfloor tubes. External down comers, 130 mm in diameter, are used to supply water from the steam drum to the water drum. Gases leaving the furnace pass over six rows of 50 mm diameter screen tubes. The boiler is encased by an all-welded double casing, and fully retractable soot blowers are fitted in the furnace and superheater section. Refer to the below Line diagram to understand the steam generation and circulation inside the boiler.

Foster wheeler ESD III boilers

The Foster Wheeler ESD III boiler has been designed to suit advanced steaming conditions with the final steam temperature controlled by an attemperator between the primary and secondary superheater passes. But this time, it is not an Air-Cooled Attemperator. The final superheat temperature is completely controlled by a water-cooled attemperator mounted in the steam drum.

It contains four to six rows of screen tubes. To meet the flame-length requirement, oil-fired burners are positioned on the rooftop. The furnace is completely water cooled by close-pitched tubes to reduce refractory maintenance.  Gas-tight all-welded monowalls are used in place of water walls. Monowall is the Foster Wheeler Trade Name for a membrane wall. These monowalls are made up of close pitched 62 mm diameter tubes. The basic construction difference between monowall and water wall is shown below.

The feedwater enters the steam drum after passing through several economizer sections. The water enters the water drum via several external down comers. A dividing water wall completely separates the upper portion of the furnace from the superheater section. Tubes are splayed out only in the bottom part to enable the gases from the furnace to pass through on their way to the multi-loop type superheater. The superheater consists of primary and secondary sections, a water-cooled attemperator mounted in the steam drum. The control valve controls the amount of steam entering the attemperator, the rest of the steam directly passes from the primary to the secondary superheater. Parallel steam flow is employed between the exhaust gas and the steam inside the secondary superheater. Therefore, the heat transfer is reduced and the temperature of the superheated steam is under control. This also reduces the formation of bonded deposits in the tubes. If you have noticed, Counterflow is employed in the previous versions of the ESD boiler. Refer to the below Line diagram to understand the steam generation and circulation inside the boiler.

Foster wheeler ESD IV boilers

The ESD IV boiler is similar in design to the ESD III. Monowalls are extensively used in ESD IV, thus forming a gas-tight boiler. There are only two rows of screen tubes, they can be easily accommodated at the headers now. All the tubes are welded directly to the headers and drums, thus avoiding internal leakage. As in the later examples of the ESD III design, the superheaters and economizers are arranged athwartships. This ensures easier support of the shorter elements. Steam temperature control is the same as in the ESD III type, by attemperator in the steam drum.

Gastight diaphragm walls of welded construction surround the furnace. Which subsequently reduces the need for refractory material, and allowing the use of a single boiler casing.

Foster wheeler ESRD boilers

ESRD stands for external superheater reheat D type boiler. For increasing the efficiency of the steam turbine system onboard the ship, we use a reheat cycle and a special type reheat boiler. Supersaturated heat from the boiler enters a high-pressure turbine and gets expanded. Then it returns to the boiler for increasing its steam energy. This is done by reheating the steam inside the boiler.

The ESRD boiler is constructed like the ESRD IV boiler. But the passage of steam from the furnace to the uptake section is divided into two paths using an additional mono wall. Reheater tubes are placed above the superheater section. The steam flow into the reheater and superheater sections is controlled by their respective dampers. Foster Wheeler developed two designs for reheat boilers in the past. They are controlled superheat and D reheat type boilers. We will not be discussing these types in this article.

Why the usage of water tube/Foster wheeler boiler is limited onboard the ship?

Water-tube boilers along with steam turbines were used for propulsion purposes in the past. Usage of steam turbines has its own merits and demerits on board the ship.

• Steam turbines are effectively used as the prime mover for alternators. But the initial cost of construction is too high.
• The thermal efficiency of steam turbines is higher when compared to reciprocating engines. But they become ineffective at low load conditions.
• The start-up of the turbines takes a quite long time.

Around 90% of world electricity generation is satisfied by steam turbines. But it is still struggling to give its comeback to the marine industry. As its use is limited onboard the ship, water tube boilers are not vastly used as compared to smoke tube boilers. We will be covering smoke tube boilers in our upcoming articles.

By

Cdt. Krishna Sai , TMI